Workers shovel debris from the top of a dock float torn loose from a Japanese fishing port by the 2011 tsunami that washed up Tuesday on Agate Beach near Newport, Ore. Workers with shovels, rakes and other tools first scraped the structure clean, then briefly used low-pressure torches to sterilize the dock. (ASSOCIATED PRESS / Oregon Parks and Recreation Department)

When the tsunami hit the northern coast of Japan last year, the waves ripped four dock floats the size of freight train boxcars from their pilings in the fishing port of Misawa and turned them over to the whims of wind and currents.

One floated up on a nearby island. Two have not been seen again. But one made an incredible journey across 5,000 miles of ocean that ended this week on a popular Oregon beach.

Along for the ride were hundreds of millions of individual organisms, including a tiny species of crab, a species of algae, and a little starfish all native to Japan that have scientists concerned if they get a chance to spread out on the West Coast.

“This is a very clear threat,” said John Chapman, a research scientist at Oregon State University’s Hatfield Marine Science Center in Newport, Ore., where the dock washed up early Tuesday. “... It’s incredibly difficult to predict what will happen next.”

A dozen volunteers scraped the dock clean of marine organisms and sterilized it with torches Thursday to prevent the spread of invasive species, said Chris Havel, spokesman for the state Department of Parks and Recreation, which is overseeing the dock’s fate.

The volunteers removed a ton and a half of material from the dock, and buried it above the high-water line, Havel said.

Biologists have identified one species of seaweed, known as wakame, that is native to Japan and has established in Southern California but has not yet been seen in Oregon, he said.

While scientists expect much of the floating debris to follow the currents to the Great Pacific Garbage Patch, an accumulation of millions of tons of small bits of plastic floating in the northern Pacific, tsunami debris that can catch the wind is making its way to North America. In recent weeks, a soccer ball washed up in Alaska, and a Harley Davidson motorcycle in a shipping container was found in British Columbia, Canada.

How the dock float — 165 tons of concrete and steel measuring 66 feet long, 19 feet wide and 7 feet high — turned up on Agate Beach, a mile north of Newport, was probably determined within sight of land in Japan, said Jan Hafner, a computer programmer in the University of Hawaii’s International Pacific Research Center, which is tracking the 1.5 million tons of tsunami debris likely floating across the Pacific.

That’s where the winds, currents and tides are most variable, due to changes in the coastline and the features of the land, even for two objects a few yards apart, he said. Once the dock float got into the ocean, it was pushed steadily by the prevailing westerly winds, and the North Pacific current.

“If you have leaves falling from a tree ... one leaf will be moving in a slightly different direction from another one,” Hafner said. “Over time, the differences get bigger and bigger and bigger.

“Something similar is happening on the ocean.”

After it came ashore, the Japanese consulate was able to track down the origin of the dock float from a plaque bolted to it commemorating its installation in June 2008. Deputy Consul Hirofumi Murabayashi said Wednesday from Portland, Ore., that it was one of four owned by Aomori Prefecture that broke loose from the port of Misawa on the northern tip of the main island during the tsunami.

Akihisa Sato, an engineer with Zeniya Kaiyo Service, the dock’s Tokyo-based manufacturer, said the docks were used for loading fish onto trucks. One of them turned up several weeks later on an island south of Misawa, but the other two remain missing.

Sen. Ron Wyden, D-Ore., called on the National Oceanic and Atmospheric Administration to redouble its efforts to track the debris, saying something as big as the dock could pose a danger to ships at sea.

NOAA’s tsunami marine debris coordinator, Ruth Yender, said if the Pacific were shrunk to the size of a football field, something like the dock would be the size of a human hair, making it very difficult to monitor, even from satellites.

The dock tested negative for radiation, which was to be expected if the dock broke loose before the nuclear power plant accident triggered by the waves, said Havel.

Chapman said the dock float was covered with masses of algae, kelp, barnacles, mussels and other organisms. One square-foot area weighed nine pounds.

“This is a whole, intact, very diverse community that floated across from Japan to here,” he said. “That doesn’t happen with a log or a thrown-out tire. I’ve never seen anything like this.”

Of particular concern was a small crab that has run wild on the East Coast, but not shown up yet on the West Coast, and a species of algae that has hit Southern California, but not Oregon. The starfish, measuring about three inches across, also appears to be new to U.S. shores.

“It’s almost certainly true that most of the things on this have not been introduced to this coast yet,” Chapman said. “We’re going to see more of these things coming.”

Tom Cleveland, a housekeeping supervisor at nearby beachfront condominiums, said people curious to see it have been jamming up traffic at a beach parking lot.

“Everybody and their brother has been here looking at it and checking it out,” Cleveland said. “Obviously, we knew things would be coming our way, but I didn’t expect anything this size.”